1. Field of the Invention
The present invention relates to valve assemblies for pressure swing adsorption systems.
2. Discussion of the Background
Pressure Swing Adsorption (PSA) is a technique used to fractionate mixtures of gases to provide at least one purified product gas and a raffinate byproduct mixture. PSA has been successfully used to separate hydrogen from other gases, oxygen and nitrogen from air, helium from natural gas, among others.
Early PSA systems generally used four adsorbent vessels operated in parallel. An example of such a PSA system is described in U.S. Pat. No. 3,430,418 to Wagner. Later improvements to Wagner's process added an additional pressure equalization step while retaining four adsorbent beds (see U.S. Pat. No. 3,564,816 to Batta), and subsequently added even more pressure equalization steps to seven or more beds (see U.S. Pat. No. 3,986,849 to Fuderer et al.). These increases in the number of pressure equalizations and the number of adsorbent vessels were implemented to increase the product recovery and the adsorbent productivity. Unfortunately, the increases in performance were also accompanied by a coincident increase in the number of valves required to operate the systems. For example, the Wagner system utilized at least thirty-one valves, the Batta system utilized at least thirty-three valves, and the Fuderer et al. system utilized at least forty-four valves.
The increase in the number of adsorbent vessels and valves in PSA systems undesirably increases manufacturing and operational costs. Many innovative cycles have been proposed that economize the number of beds and/or valves employed in PSA systems. An excellent example of such a system is described in U.S. Pat. No. 3,738,087 to McCombs, as well as a later process described in U.S. Pat. No. 4,194,890 to McCombs. These patents describe PSA systems with as few as two adsorbent vessels; however, continual delivery of product is usually impossible or can be achieved only at a reduced product pressure. Furthermore, these sorts of cycles are generally understood to offer lower product gas recovery and adsorbent utilization at a given set of feed conditions. Efforts to produce more complex cycles with fewer valves, or at least simpler plumbing arrangements than that of Wagner, Batta, and Fuderer et al. while maintaining their high performance have been revealed in U.S. Pat. No. 4,761,165 to Stöcker and in U.S. Pat. No. 6,146,450 to Duhayer et al.
Several PSA systems have been presented that reduce mechanical complexity through the implementation of rotary valving arrangements by combining many valve functions from earlier processes to reduce complexity. Examples of such systems include U.S. Pat. No. 4,272,265 to Snyder, U.S. Pat. No. 4,925,464 to Rabenau et al., and U.S. Pat. No. 6,063,161 to Keefer et al. In each case the use of one or more rotating assemblies with valving functionalities are employed in place of one or more independent valves. Although these methods advantageously reduce the plumbing complexity compared to independent valves plumbed in a traditional manner, they have several undesirable features. First, they fix the relative duration of the various PSA cycle steps, and are thus unable to respond to changes in flow conditions to optimize operation with variability in feedstock composition, temperature, pressure or flowrate. Keefer et al. describe the addition of special secondary valves to their basic rotary valves in order to fine tune the PSA cycle, which undesirably increase complexity and are not adjustable during operation. Second, all rotating valves rely on sliding sealing surfaces to separate purified product from impure feed or waste gases. Indeed, Keefer et al. teach elaborate mechanical steps needed to overcome this potential limitation to product purity. Sliding seals are more difficult to maintain, provide worse sealing, and are more susceptible to damage due to particle contamination than simple contact seals without sliding. Finally, the rotating valve arrangements make very complex cycles difficult to execute because of the complexity of the rotary valve porting arrangements required for their implementation. These valves have, therefore, chiefly been used to implement simple cycles with relatively low product recovery and adsorbent utilization compared to the most advanced cycles taught in the art.
An additional feature present in many PSA cycles of the art is the use of counter-current blowdown of an adsorbent vessel with purified product gas. In early cycles such as that of Batta or Fuderer et al., this was accomplished by providing an independent product gas manifold maintained at a low pressure via a pressure regulating valve or throttling device, with an independent actuated valve provided for each adsorbent vessel. Alternatively, some simple cycles were provided with a flow control valve connecting the product manifold to each vessel. An example of this method is described in U.S. Pat. No. 4,194,890 to McCombs. This simplified method has the disadvantage that the flow of product gas through the vessel cannot be independently controlled, which leads to a reduction in product recovery as compared to the traditional methods. A second improved approach using proportionally-controlled valves was taught by Stöcker. Although the method of proportional control of the product delivery valve does desirably reduce the number of plumbing connections relative to the art, and provides the ability to stop flow entirely at some stages in the cycle, proportional valves may suffer from lower reliability and higher cost than on-off valves.
The inventors of the present invention have determined that none of the pressure swing adsorption systems described in the above patents teaches any fundamental mechanical deviations from traditional construction using separately plumbed valves. The inventors have determined that the use of independently connected valves is highly undesirable, since each valve requires at least two plumbing connections. These connections are often made with expensive fittings, or through welding to ensure product purity and/or to prevent leakage of noxious or flammable process gases. This proliferation of fittings undesirably increases manufacturing expense, increases the packaged system volume, and reduces safety and reliability due to the possibility of leakage.
The proliferation of plumbing, and the attendant volume required for packaging, is further complicated by the requirement to provide mechanical support to the adsorbent vessels. The plumbing and valving, due to its relatively great mass, may exert considerable forces on the pressure vessels unless all are carefully designed and well-supported. The provision of such supports disadvantageously increases the system mass, volume, and manufacturing cost. Further, the adsorbent vessels, which are subject to fatigue failure due to the cyclic nature of the pressure stresses, are inherently difficult to support structurally without further increasing their weight to compensate for high localized stresses.